Scenario
During planned fail-over testing, Veeam was unable to communicate with a vCenter at a remote site whilst the active directory controllers and DNS server at that site were shutdown.
Other active directory controllers and DNS servers were available at the local site where Veeam Mgmt server is located.
This was unexpected.

Can anyone explain what's gone wrong here?
Surely Veeam Mgmt server and proxies should still be able to connect to vCenter ?

Veeam Mgmt Server at Secondary site
Multiple Veeam proxies at both primary and secondary sites
Multiple DNS and Active directory servers at both sites.
vCenters at both sites
Only AD / DNS servers at primary site were shutdown at this point.
Local AD and DNS at secondary site still available.
Account used for Veeam connection to all vCenters was administrator@vsphere.local
vSphere web client from Veeam Mgmt server and proxies continued to work and login with AD or Local accounts.
Veeam console could browse inventory for both vCenters.

Only SOAP connection to primary site vCenter seemed unable to connect.

@HannesK, Normally the forum is good for asking general questions on usage and functionality. Support does a decent job on resolving specific errors, but increasingly seems unwilling to answer questions.
As above, I asked a specific question and rather than answering the question you posted a link to a very long and all encompassing article.

@Andreas, this client has restrictions on uploading to the Internet, so successfully extracting Gigabytes of logs from a secure network and then transferring to a location from whence they can be uploaded is a complex and time consuming tasks.